A solid dioxouranium(VI) complex of sodium 1,2-naphthoquinone-1-monoxime-7-sulfonate (HL) has been synthesized and characterized. According to elemental analysis its metal to ligand molar ratio was 3. The analysis of potentiometric data obtained from a simulated synthesis by the MINIQUAD program showed that the main component of synthetic product was (UO2)3(OH)4L although the species (UO2)3(OH)5L made up about one fifth of the product. TG, DTG and DSC techniques have been used at 39-900-degrees-C to analyze the thermal decomposition of the synthetic product in dynamic air and nitrogen atmosphere. The gaseous decomposition products were identified at 50-400-degrees-C by the pyrolysis-MS technique. Some weakly bonded water was eliminated before 90-degrees-C; the product also contained 4.8 moles coordinated water which was released at 90-260-degrees-C. The decomposition proceeded at 260-800-degrees-C in several, partially simultaneous processes, releasing CO2 together with some minor species. The residues, containing mostly U3O8, were 75.1% and 78.7% of the dry mass of the sample in dynamic air and nitrogen atmosphere, respectively. The theoretical amount of U3O8 is 72.0%.